Generic transuranic waste is defined to be nuclear waste of unknown composition that principally includes .sup.233 U, .sup.235 U, .sup.239 Pu, .sup.240 Pu, .sup.241 Am, .sup.244 Cm, and .sup.252 Cf, in addition to other isotopes present in much smaller concentrations. The growth of the nuclear industry has given rise to the need for an interrogation system to perform rapid, quantitative assays of low fissile content wastes and scraps contained in high or low density matrices. The present invention relates generally to quantitative assay of generic transuranic wastes and more particularly to pulsed thermal-neutron interrogation of samples for the presence of fissile materials, combined with performance of a sequence of active and passive neutron measurements which yields accurate and sensitive assay values of up to seven simultaneously contained transuranic waste isotopes.
The apparatus and method of the instant invention measures prompt and delayed fission neutron yield and counts the coincidence of emitted prompt neutrons produced as a result of active neutron interrogation. With these three independent attributes of thermal neutron fission, one can uniquely determine the masses present of three or fewer fissile isotopes contained in a package such as a 208 l barrel of transuranic waste. Indeed, there are only three important fissile isotopes; .sup.233 U, .sup.235 U and .sup.239 Pu. Further, measurement of the passive coincident neutron yield and multiplicity and the passive noncoincidence neutron yield, all resulting from natural spontaneous fission processes which release neutrons or from spontaneous alpha particle emission which particles produce neutrons upon colliding with oxygen nucleii present in the sample, allows the determination of up to four additional isotopes contained in the sample to be analyzed. Therefore, by solely measuring neutrons one can analyze a sample containing up to three fissile isotopes and four non-fissile isotopes with 1 mg sensitivity for .sup.239 Pu or .sup.235 U in standard 208 l barrels filled with a variety of common waste and scrap materials.
U.S. Pat. No. 3,786,256 issued to Samuel Untermeyer on Jan. 15, 1974 describes a method and apparatus for nuclear fuel assay with a neutron source and coincident fission neutron detectors. Therein he teaches coincident neutron and gamma emission measurements, obtaining the required discrimination from thermal interrogation neutrons and the desired fission neutrons by this procedure. The method of the instant invention, on the other hand, does not require any gamma emission measurements (and in fact our detectors are quite insensitive to gamma radiation), and although teaching coincidence neutron measurements as independent observations which enter the algorithm for quantitative determination of various isotopes, also teaches both time-resolved single neutron measurement of fast fission neutrons, and total passive neutron flux determination. Untermeyer's apparatus and method are stated to be useful in both the passive mode and with neutron interrogation. Our invention provides an analytical procedure for up to three simultaneously contained fissile isotopes. Untermeyer's allows for one only. Measurement of the total passive neutron lux allows four non-fissile isotopes to be quantitatively determined by our method, while Untermeyer can only determine three such isotopes. Moreover, Untermeyer does not teach combining the active and passive neutron measurements to simultaneously evaluate both fissile and non-fissile components in a waste sample. Untermeyer's apparatus utilizes organic scintillators which are sensitive to both neutrons and gamma radiation the detection of which is taught. The apparatus of the instant invention utilizes .sup.3 He detectors which are gamma insensitive and necessarily that way to enable the collected counts to be relatable to the number of neutrons present especially for highly radioactive samples. Finally, Untermeyer teaches the use of a one at a time neutron emission neutron source to avoid false counts, whereas our invention allows many interrogation neutrons to be used simultaneously, the necessary discrimination being derived from the use of a pulsed interrogation neutron source and specially designed .sup.3 He neutron detection packages. The pulsed interrogation source allows us to use time domain discrimination between interrogating neutrons and fast fission signal neutrons. The .sup.3 He neutron detection packages allow us to detect signal neutrons with high sensitivity while rejecting interrogation neutrons with a rejection factor of 10.sup.8.
"A 1nCi/g Sensitivity Transuranic Waste Assay System Using Pulsed Neutron Interrogation," by W. E. Kunz, J. D. Atencio, and J. T. Caldwell was published on Nov. 1, 1980 in Proceedings, 21st Annual Meeting of The Institute of Nuclear Materials Management in Palm Beach, FL, June 30-July 2, 1980, Vol. IX, page 131. Therein the authors describe an apparatus for determining the total amount of fissile material present in a transuranic waste sample. The apparatus is designed to detect prompt fission neutrons to the exclusion of the thermalized interrogation neutrons and any delayed neutron emission. To obtain the increased sensitivity to fission neutrons, heavy detector shielding is used to stop all but the fast neutrons, thereby significantly reducing the neutron background, but in so doing losing information critical to the instant apparatus and method. One major change has been made in the apparatus of applicants' invention. Although Kunz et al. mention the use of a bare, low pressure .sup.3 He internal flux monitor designed to keep track of the interrogation neutron flux, and applicants' invention describes several unshielded detectors which measure the passive neutron emission and the delayed neutron production, both important for the isotopic assay of the instant method, the low pressure detector taught by Kunz is designed to be operated in high neutron fluxes and is therefore too insensitive to be useful for observing delayed neutrons which the authors teach away from doing.
Thus by including the measurement of the total neutron flux for both active and passive interrogations, and delayed neutron emission for active interrogations, in addition to the coincidence neutron emission suggested by Untermeyer, one can quantitatively determine the amounts of three fissile isotopes and four non-fissile isotopes at sensitivities of at least 10 nCi/g, as opposed to Untermeyer's one fissile and three non-fissile isotope analysis for which he does not quote sensitivity limits (practical instrumentation based on the Untermeyer concept generally has a fissile assay sensitivity 1000 times poorer than that obtained with practical instrumentation using the instant concept). The reason for the improvement rests on the fact that our invention utilizes a greater number of independent measurements, thereby increasing the number of variables we can uniquely solve for and uses time-domain (pulsed neutron interrogation) measurements which significantly increase sensitivity. The use of a pulsed interrogating neutron source arose from the realization that cadmium shielded moderated .sup.3 He detectors are very sensitive devices for discriminating between thermal interrogation and fast fission neutrons, that delayed neutron intensity was also of great utility, and finally that pulsed thermal fission coincidence neutron detection could be easily used to glean additional information from the sample.
Fortunately, it turns out that the major non-fissile isotopes present in 99% of transuranic wastes from nuclear power reactors in the United States are four in number (.sup.241 Am, .sup.252 Cf, .sup.244 Cm and .sup.240 Pu; any .sup.242 Pu present being lumped together with the .sup.240 Pu). Further, the major fissile isotopes of interest in the waste material are .sup.233 U, .sup.235 U, and .sup.239 Pu; any .sup.241 Pu present affects the resulting neutron counts by less than 1%. Our method then allows the simple, non-destructive quantitative analysis of most waste materials from nuclear power and weapons materials production reactors which was not previously possible.